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Fluids, Shear Zones and Continental Rheology Bruce Yardley, School of Earth Sciences, University of Leeds The strength of quartz-bearing crystalline rocks is very strongly dependent on the presence or absence of water. During progressive heating and metamorphism, continental rocks are generally believed to contain trace amounts of fluid at near-lithostatic pressure, giving high water activities and ductile behaviour from relatively low temperatures. However this is a rare geological setting, and continental rheology in general is dominated by the behaviour of old crystalline rocks that are at a lower temperature than that of their original formation. Hence the thermal history of continental crust is arguably the most important influence on its rheology. Simple equilibrium calculations, coupled with kinetic considerations, demonstrate that the fugacity of water (and other volatile species) in crystalline rocks of the middle and lower continental crust is extremely low, and is not consistent with the presence of a free fluid phase. Any fluid that successfully infiltrates will be rapidly consumed by retrograde reactions. Fluid inclusion studies of veins in basement rocks adjacent to the Oslo graben demonstrate precisely this process in operation as sedimentary fluids penetrated basement in a fluid-starved environment, although the physical mechanism is not entirely clear. At temperatures of a few hundred degrees, silicate rocks are weak and ductile when wet, but strong if dry. Fluid infiltration to dry crust will lead to localised weakening and consequent deformation (i.e. shear zones), but this will be followed by incorporation of fluid into mineral lattices, and hence strengthening of the rock mass once more. As a result, from the perspective of a relatively small rock mass, deformation will be sporadic as rock strengths alternate between those of wet and dry rock in a relatively uniform stress field. In this situation, rheology is a time-dependent variable. Viewing a larger crustal volume however, it may be possible to take a continuum approach in which deformation is localised into very small parts of the total volume at any one time, and within those zones only, the rheology of wet rock is an appropriate descriptor. The main obstacle to progressing this approach to crustal deformation further, is the difficulty in understanding and predicting the infiltration of water into the middle and lower crust.